Most small compressors for refrigeration applications require a PTC thermistor in line with the compressor start/auxiliary motor winding in order to provide a phase shift between the start/auxiliary motor winding and the main motor winding as well as a means for reducing the start/auxiliary winding current flow as the compressor motor reaches its operating rotational speed. It is possible for the thermistor (commonly referred to as a pill) to enter a state called “thermal runaway,” a failure mode during which the thermistor resistance significantly exceeds its typical steady-state (self-regulated) resistance and passes its maximum resistance (RMAX) value, after which its resistance decreases and the thermistor heats up at an uncontrolled rate due to the increasing resultant current. One possible thermistor failure mode is fracture, which occurs due to thermal stress.
U.S. Pat. No. 6,172,593, assigned to Murata Manufacturing Co., Ltd., describes a prior art device with spring contacts and non-conductive support posts which maintain contact with the thermistor throughout the normal usable life of the thermistor. Because thermistors used for starting refrigerator compressors typically self-regulate at 160° C.-170° C. during the switched (high-resistance) state, materials in intimate contact with the thermistor electrodes have to have a higher relative temperature index (RTI) which are more expensive. In addition, this embodiment places portions of the thermistor in tension and compression on opposing sides of the pill with those forces reversed in direction near the other end of the thermistor. Unfortunately, under some circumstances if the pill doesn't crack in a manner in which current flow is stopped, fractured portions of the pill (also referred to as rubble) remain in electrical contact with internal terminals which can cause arcing and overheating.
In refrigerator applications there is a need for a more reliable, lower cost design which minimizes electrical arcing, current flow and subsequent overheating upon failure of the pill. Conventional thermistor devices do not always fail in a manner which eliminates excessive overheating and electrical arcing upon failure of the thermistor.